Suppose they didn't require that. Wouldn't that open themselves up to DDoS? An angry nation or ransom-seeker could direct bots to create accounts and stuff them with noise.

I think the deal is you marry the strong crypto with a human mediated security process which provides high confidence the message sender maps to the human you think they are. And even if they are, they could be a narc. Nothing in strong crypto prevents narcs in whom ill-advised trust has been granted from copying messages they're getting over the encrypted channel and forwarding them to the man.

And even then, a trusted participant could not understand they're not supposed to give their private keys out or could be rubber-hosed into revealing their key pin. All sorts of ways to subvert "secure" messaging besides breaking the crypto.

I guess what I'm saying is "Strong cryptography is required, but not sufficient to ensure secure messaging."

Yes. Cheap–identity systems such as Session and SimpleX are trivially vulnerable to this, and your only defence is to not give out your address as they are unguessable. If you have someone's address, you can spam them, and they can't stop it except by deleting the app or resetting to a new address and losing all their contacts.

SimpleX does better than Session because the address used to add new contacts is different from the address used with any existing contact and is independently revocable. But if that address is out there, you can receive a full queue of spam contacts before you next open the SimpleX app.

Both Session and SimpleX are trivially vulnerable to storage DoS as well.

There are a lot of solutions to denial of service attacks than to collect personal information. Plus, you know, you can always delete an account later? If what Signal says is true, then this amounts to a few records in their database which isn't cause for concern IMO